Most of us "builders" buy a complete upper or one minus the BCG and charging handle, then find those parts. Also a quality milspec stripped lower, LPK and milspec receiver extension kit and milspec diameter stock of your choice. Then of course add a sling and rear sight. And front sight if your upper doesn't have the standard A2 FSB.

Lately PSA has been the bang for the buck brand around here. BCM is a great choice and is considered a cut above most others in its price range.

+1

PSA well have uppers on a "black friday" sale, under $300 for mil-spec upper less BCG and CH. CORE15 has BCG and CH at great prices.

I have two local firearm forums with a lot of used AR stuff, a few good deals can be found. AR prices are up a lot from one year ago.